Over the years, Nd—Fe—B sintered magnets find an ever increasing range of application including hard disk drives, air conditioners, industrial motors, power generators and drive motors in hybrid cars and electric vehicles. When used in air conditioner compressor motors, vehicle-related components and other applications which are expected of future development, the magnets are exposed to elevated temperatures. Thus the magnets must have stable properties at elevated temperatures, that is, be heat resistant. The addition of Dy and Tb is essential to this end whereas a saving of Dy and Tb is an important task when the tight resource problem is considered.
For the relevant magnet based on the magnetism-governing primary phase of Nd2Fe14B crystal grains, small domains which are reversely magnetized, known as reverse magnetic domains, are created at interfaces of Nd2Fe14B crystal grains. As these domains grow, magnetization is reversed. In theory, the maximum coercive force is equal to the anisotropic magnetic field (6.4 MA/m) of Nd2Fe14B compound. However, because of a reduction of the anisotropic magnetic field caused by disorder of the crystal structure near grain boundaries and the influence of leakage magnetic field caused by morphology or the like, the coercive force actually available is only about 15% (1 MA/m) of the anisotropic magnetic field.
It is known that the anisotropic magnetic field of Nd2Fe14B is significantly enhanced when Nd sites are substituted by Dy or Tb. Accordingly, substitution of Dy or Tb for part of Nd leads to an enhanced anisotropic magnetic field and hence, an increased coercive force. However, since Dy and Tb cause a significant loss of saturation magnetization polarization of magnetic compounds, an attempt to increase the coercive force by addition of these elements is inevitably followed by a decline of remanence (or residual magnetic flux density). That is, a tradeoff between coercivity and remanence is unavoidable.
When the magnetization reversal mechanism as mentioned above is considered, if part of Nd is substituted by Dy or Tb only in proximity to primary phase grain boundaries where reverse magnetic domains are created, then only a low content of heavy rare earth element can increase the coercive force while minimizing a decline of remanence. Based on this idea, a method of preparing an Nd—Fe—B magnet known as two-alloy method was developed (see JP 2853838). The method involves separately preparing an alloy having a composition approximate to Nd2Fe14 B compound and a sintering aid alloy having Dy or Tb added thereto, grinding and mixing them, and sintering the mixture. However, since the sintering temperature is as high as 1,050 to 1,100° C., Dy or Tb is diffused inward of primary phase crystal grains of about 5 to 10 μm from their interface to a depth of about 1 to 4 μm, with a concentration difference from the center of primary phase crystal grains being not so large. For achieving a higher coercive force and remanence, it is ideal that heavy rare earth element be enriched in a higher concentration in a thinner diffusion region. It is important for heavy rare earth element to diffuse at lower temperature. To overcome this problem, the grain boundary diffusion method to be described below was developed.
In the literature, the phenomenon was discovered in 2000 that when a thin magnet piece of 50 μm is coated with Dy by sputtering and heat treated at 800° C. so that Dy is enriched in grain boundary phase, the coercivity is increased without a substantial loss of remanence. See K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteenth International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000). The same phenomenon was confirmed in 2003 when a magnet body of several millimeters thick was coated with Tb by three-dimensional sputtering. That is, the phenomenon is applicable to magnet bodies of practically acceptable size. See S. Suzuki and K. Machida, “Development and Application of High-Performance Minute Rare Earth Magnets,” Material Integration, 16, 17-22 (2003); and K. Machida, N. Kawasaki, S. Suzuki, M. Ito and T. Horikawa, “Grain Boundary Modification and Magnetic Properties of Nd—Fe—B Sintered Magnets,” Proceedings of Japan Society of Powder & Powder Metallurgy, 2004 Spring Meeting, p. 202. These methods based on grain boundary diffusion involve once preparing a sintered body, supplying Dy or Tb to the surface of the sintered body, letting the heavy rare earth element diffuse into the sintered body through the grain boundary phase which is a liquid phase at a temperature lower than the sintering temperature, for thereby substituting a high concentration of Dy or Tb for Nd only in proximity to the surface of primary phase crystal grains.
In the case of coating, typically three-dimensional coating, by sputtering, a relatively large size system is necessary. Feeds to the system must be fully clean. After the system is charged, a high vacuum must be maintained. The coating step is thus a time and labor-consuming operation including the time taken until the predetermined thickness is reached. Since magnet pieces having metallic Dy or Tb coated by sputtering tend to fuse together, they must be spaced apart during heat treatment for diffusion. It is difficult to charge the heat treatment furnace with the number of magnet pieces compliant with its capacity, resulting in low productivity.
Various modifications of the grain boundary diffusion method have been proposed for mass-scale production. These methods differ mainly in the supply of Dy or Tb (to be diffused) to the magnet. The inventors previously proposed in JP 4450239 (WO 2006/043348) a method involving immersing a sintered body in a slurry of a powder fluoride or oxide of Dy or Tb in water or organic solvent, taking out the sintered body, drying and heat treating for diffusion. During the heat treatment, the Nd-rich grain boundary phase is melted and part thereof is diffused to the sintered body surface, with substitution reaction between Nd and Dy/Tb taking place between the diffused part and the coated powder, through which Dy/Tb is incorporated into the magnet.
Besides, a method involving mixing Dy or Tb fluoride with calcium hydride, coating the mixture, heat treating for thereby reducing the fluoride into the metal and letting the metal diffuse is proposed in JP 4548673 (WO 2006/064848). Another method involves admitting Dy metal/alloy to a heat treating box, and effecting diffusion treatment for letting Dy vapor diffuse into the magnet as disclosed in JP 4241890, WO 2008/023731; K. Machida, S. Shu, T. Horikawa, and T. Lee, “Preparation of High-Coercivity Nd—Fe—B Sintered Magnet by Metal Vapor Sorption and Evaluation,” Proceedings of the 32nd Meeting of Japan Society of Magnetism, 375 (2008); Y. Takada, K. Fukumoto, and Y. Kaneko “Effect of Dy Diffusion Treatment on Coercivity of Nd—Fe—B Magnet,” Proceedings of Japan Society of Powder & Powder Metallurgy, 2010 Spring Meeting, p. 92 (2010); K. Machida, T. Nishimoto, T. Lee, T. Horikawa and M. Ito, “Coercivity Enhancement of Nd—Fe—B Sintered Magnet by Grain Boundary Modification Using Rare Earth Metal Fine Powder”, Proceedings of Japan Institute of Metals, 2009 Spring Meeting, 279 (2009). Coating of metal powder (metal element, hydride or alloy) is disclosed in JP-A 2007-287875, JP-A 2008-263179, JP-A 2009-289994, WO 2009/087975, and N. Ono, R. Kasada, H. Matsui, A. Kouyama, F. Imanari, T. Mizoguchi and M. Sagawa, “Study on Microstructure of Neodymium Magnet Subjected to Dy Modification Treatment,” Proceedings of Japan Instituted of Metals, 2009 Spring Meeting, 115 (2009).
Studies are also made on the mother alloy amenable to coercivity improvement by grain boundary diffusion, that is, anisotropic sintered body prior to grain boundary diffusion. The inventors discovered in JP-A 2008-147634 that a significant coercivity enhancement effect is achievable by providing Dy/Tb diffusion routes. Based on the belief that potential reaction of diffused heavy rare earth element with Nd oxide within the magnet causes to reduce the diffusion amount, it was proposed in JP-A 2011-82467 to gain a certain diffusion amount by previously adding fluorine to the mother alloy to convert the oxide to oxyfluoride for reducing reactivity with Dy/Tb. It has never been proposed to improve diffusion efficiency while paying attention to the chemical properties of the Nd—rich grain boundary phase affording diffusion routes or the Nd2Fe14B compound eventually undergoing substitution reaction on the surface.